The biological cycle of rock-derived nutrients on the continents is a major component of element transfer between the Earth’s surface compartments, but its magnitude currently remains elusive. In an attempt to close this knowledge gap, we examine the difference between the elemental supply to catchments through rock degradation and the corresponding elemental riverine export, for two non-nutrient elements lithium (Li) and sodium (Na) and two nutrients-like elements potassium (K) and barium (Ba), in the Amazon River basin as well as 20 of the largest world river catchments. Overall, we show that the riverine export of K and Ba are lower than their estimated release by catchment scale rock degradation, while the two fluxes match for Li and Na. Barium isotope constraints lending support to this observation, we take this difference between these two element groups as a suggestion of the influence of biological uptake of rock-derived nutrients on river chemistry. Nevertheless, the magnitude of riverine K depletion cannot be reconciled with a pervasive growth of the biota on continents, nor with an “occult” export of organic material that would go unnoticed by common sampling protocols. One plausible explanation for this conundrum could lie in the complex partitioning of elements amongst soil, biota, and dead organic matter. As a consequence, our study emphasises the need for further work aiming at deciphering the cycle of rock-derived nutrients in the Critical Zone.
